Catalytic hydrogenation is a fundamental reaction in chemistry, and is used in a large number of chemical processes. Catalytic hydrogenation of ketones and aldehydes are useful and indispensable processes for the synthesis of alcohols, which are valuable end products and precursor chemicals in the pharmaceutical, agrochemical, flavor, fragrance, material and fine chemical industries.1 
To achieve a catalytic hydrogenation transformation in the reduction of ketones and aldehydes, molecular hydrogen (H2) is used. However, for the hydrogenation process to proceed, a catalyst or catalytic system is needed to activate the molecular hydrogen.
Noyori and co-workers developed the versatile RuCl2(PR3)2(diamine) and RuCl2(diphosphine)(diamine) hydrogenation catalyst system that are highly effective for the hydrogenation of ketones.2 It was subsequently discovered that the Noyori catalysts were effective for the reductive hydrogenation of imines to amines.3 It has been determined that in the presence of a base and hydrogen gas that the active catalyst species are ruthenium dihydride species of the type RuH2(PR3)2(diamine) and RuH2(diphosphine)(diamine).3,4 The mechanistic investigation included the synthesis, isolation and characterization of the highly reactive amidoaminohydrido species that activates hydrogen to regenerate the active dihydride catalysts.4 It was clearly demonstrated that the carbonyl molecular recognition motif of these catalysts is the mutually cis N—H and Ru—H moieties in the dihydride catalysts, which facilitate hydrogenation through an outer-sphere hydrogen transfer process.4 
The synthesis and characterization of a series of iridium hydride species has recently been reported.5 This series included IrH2Cl[(iPr2PC2H4)2NH] (1), IrH3[(iPr2PC2H4)2NH] (2) and IrH2[(iPr2PC2H4)2N] (3).5 It was demonstrated that 2 and 3 are very active catalysts for the base-free transfer hydrogenation of ketones in 2-propanol, while 1 is air-stable and inactive as a catalyst in the absence of a base. It was also reported that 3 rapidly formed 2 upon exposure of a solution of 2 to hydrogen gas. Further it was reported that 1, 2 and 3 could not effect the hydrogenation of ketones in the presence of H2 at 3 atm in benzene at room temperature.5 